The lower continental crust may receive mantle-derived CO 2H 2O fluids if the Moho temperature is ≳ 660°C. Below this temperature a harzburgitic sub-continental mantle is devoid of a free fluid phase. A free fluid phase may be present only locally along faults, thrust and shear zones in the upper mantle beneath normal continental areas (e.g., shields, Hercynian crust of Central Europe). In such zones the orthopyroxene + olivine assemblage is replaced by various assemblages involving hydrates (serpentine, talc) and/or magnesite. The temperature limit of 660°C for fluid-present conditions in a harzburgitic mantle is independent of Moho depth (pressure). At temperatures of > 670°C the Moho is generally permeable for mantle fluids. The composition of this fluid is effectively controlled by the assemblage orthopyroxene + olivine in the upper mantle under fluid-present conditions. The maximum CO 2 content of a fluid phase which can be transferred from the mantle to the lower crust depends on both Moho temperature and depth (pressure). Generally, maximum CO 2 content increases with T and decreases with P (depth). At 700°C and ∼ 30-km Moho depth (8 kbar), a fluid transferred from the mantle may contain max. 40 mole% CO 2. This is not sufficiently CO 2-rich to explain the formation of low-temperature granulites by a process of dehydration of the lower crust as a result of the interaction with mantle-derived fluids.